The present invention relates to an optical functional element having a function of EXCLUSIVE-OR useful for optical information processing, that is, an optical EXCLUSIVE-OR element.
As a conventional integrated optical functional element for EXCLUSIVE-OR, as shown in FIG. 1, there is suggested an InGaAsP-type optical logic element having a structure where two optical erasable photonic parallel memories (OEPPM) 103 (refer to IEEE Electron Device Letters, VOL.11, No. 10, pp. 442-444, 1990, K. Matsuda et al.) are connected in parallel (refer to IEJCE Technical Report, QQE90-156, Adachi, Matsuno, Chino, Shibata). The above mentioned OEPPM 103 comprises an equivalent circuit, as shown in FIG. 1, having a heterojunction-type phototransistor (HPT) 102 as a basic structural element and a photonic parallel memory (PPM) element 101, connected in parallel. FIG. 2 is a schematic plan view of an example of the above mentioned element. In this element, two OEPPMs 103a and 103b each OEPPM comprising the PPM element 101 and the HPT 102 connected in parallel, are adjacently provided on the same substrate. Two HPT-PPT portions, independently biased, are arranged as they are located side by side. The element has four input or input-output windows indicated by shaded areas in FIG. 2.
Two output lights are output when a first input light 105 is incident upon the input window of each of the PPM 101a and HPT 102b, and a second input light 106 is incident upon the input window of each of the PPM 101b and HPT 102a. By superimposing these two lights so as to perform an OR operation, eventually, an EXCLUSIVE-OR can be obtained.
However, with an EXCLUSIVE-OR element of this structure, each of the first light 105 and the second light 106 are required to be supplied to a different window. Additionally, because the results of the EXCLUSIVE-OR operation are not output from one window, an alignment of an optical system is not easily maintained. Further, a space for four elements is required to perform one EXCLUSIVE-OR operation, which is a disadvantage for integration due to a large number of basic elements to be used. Furthermore, the manufacturing process becomes complex due to a fact that it is needed to form adjacent elements, each of which has a different layer structure.
FIG. 3 is a partially cutaway view of an element structure suggested by the applicant in Japanese Laid-Open Patent Application No.3-274030.
The optical functional element is formed, in turn, with a first conduction-type semiconductor substrate 111, a first conduction-type semiconductor layer (emitter layer) 112 having an energy gap wider than that of the substrate 111, a second conduction-type semiconductor layer (base layer) 113 having an energy gap wider than that of the emitter layer 112, and a first conduction-type semiconductor layer (collector layer) 114 having an energy gap the same as that of the base layer 113. The emitter layer 112, the base layer 113, and the collector layer 114 comprise a light receiving portion 122. On the collector layer 114, a first conduction-type semiconductor layer 115 having an energy gap wider than that of the substrate 111 and a second conduction-type semiconductor layer 116 having an energy gap the same as that of the layer 115 are, in turn, provided to form a light emitting portion 123. On the light emitting portion 123, a second conduction-type semiconductor layer (cap layer) 117 and a second electrode 118 are, in turn, provided. The reverse side of the substrate 111 is provided with a first electrode 110. Additionally, an optical input-output window 119 is provided on the second electrode 118 and the cap layer 117 so that the semiconductor layer 116 is exposed through the window 119.
As used above, the optical functional element designates an optoelectronic integrated functional element having an optical feedback, which element comprises a structure having alternately repeated layers of a first conduction-type semiconductor and a second conduction-type semiconductor, such as a pnpn structure or a npnp structure. Hereinafter, the optic-electronic integrated functional element is called an optical functional element.
A power source is connected to the optical functional element via a proper load resistor and a light is supplied to the optical functional element. FIGS. 4-6 are graphs showing relationships between light input and light output. The vertical axis and the horizontal axis of each graph represent the light intensity of the optical output and light input respectively. The operation mode of the relationship between optical input and output changes depending upon the resistance of the load resistor, the voltage of the power source, the inherent internal resistance of the element and the optical feedback rate. FIG. 4 is a graph showing the relationship between light input and output when the element is in an optical memory mode, FIG. 5 is in a optical bistable mode, FIG. 6 is in an optical differential gain mode. A light is emitted from the same input-output window 119 in the above three modes.
In the optical memory mode, the element is turned on when a light exceeding certain intensity is input, and the on-state is maintained even if the light input then becomes zero. In the optical bistable mode, the element has a characteristic such that the relationship between a light input and a corresponding light output during an increase of the light input is different from that during a decrease of the light input. In the optical differential gain mode, a small amount of change intensity of a light input results in a large amount of change in intensity of a light output. The operation of such modes is called threshold operation and logic operation including Boolean OR and AND can be executed.
However, in order to complete a logic operation, inverting logic operations such as NOT and EXCLUSIVE-OR (XOR) are needed. EXCLUSIVE-OR is represented by the following logic equation using AND, OR and NOT( ). EQU A xor B=(NOT(A) and B) or (A and NOT(B)).
The above mentioned logic operation element for EXCLUSIVE-OR which comprises a combination of OEPPMs is a realization of this logic equation. However, the problems in element structure and alignment of an optical system exist as formerly mentioned.